Near-UV (300-380 nm) irradiation of oxygenated, aqueous solutions of nucleosides in the presence of 2'-acetylformanilide causes degradation of substrate nucleoside with concomitant formation of hydrogen peroxide. The photosensitizer, 2'-acetylformanilide, serves as a model for the electronically similar N'-formylkynurenine, a commonly occurring tryptophan metabolite as well as tryptophan photooxidation product. Both singlet oxygen and superoxide have been found to be involved in the photochemical processes. The proposed work will involve isolation and identification of the photoproducts formed from thymidine and guanosine, as well as from adenosine (less sensitive than guanosine) and uridine (less sensitive than thymidine). Using thymidine and guanosine, the mechanisms leading to nucleoside alteration (degradation) and to hydrogen peroxide formation will be investigated. Particular attention will be paid to the possible involvement of excited state (singlet) oxygen and of superoxide as reactive species in the photo-promoted process. Chemical experiments will be carried out to determine the nature of the near-UV chromophore(s) involved in the observed synergistic lethality of near-UV radiation and H2O2 on cells and on bacteriophage. The involvement of H2O2 oxidation of a sulfur functionality will be one of the possibilities examined.